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In the North End yesterday, signs of the summer swelter were unmistakable.

At Boston Hardware on Fleet Street, standing fans and box fans flew off the shelf. The buzz of window air-conditioning units droned through most every sleepy cobblestone side street. And the chalkboard sidewalk sign at Benevento’s on Salem Street featured two very important words: Air Conditioned. That all-important amenity may come in handy today, when temperatures, which were oppressive yesterday, turn positively broiling.

While the temperature at Logan International Airport reached 91 degrees yesterday, forecasters expect an even stickier seasonal blast today. The high will be 96 degrees, but it will feel like 102 due to the humidity, said Rebecca Gould, a meteorologist with the National Weather Service.

The mayor’s office reminded residents that children and elderly people are particularly susceptible to the ill effects of summer temperatures, recommending that they drink plenty of water, wear light colors, limit physical activity, and apply sunscreen if spending time outdoors.

The National Weather Service placed all Massachusetts, except for the east slopes of the Berkshires and the Cape and Islands, on an excessive heat watch for the next two days. The Environmental Protection Agency announced that unhealthy air is predicted for Springfield, Worcester, parts of New Hampshire, and all Connecticut over the next several days.

Nicole Mustacchio, a clerk at Hanover News in the North End, has seen first-hand the effects of the sweltering temperatures.

“Every single person that has walked into this store today has told me how oppressively hot it is,’’ said Mustacchio, 21, as she sipped an iced coffee.

Chances are good that there will be an official heat wave; temperatures must be in the 90s for three consecutive days to qualify.

Mark Neally, manager at The Boston Gardener in Roxbury, said he noticed elderly people making a pit stop in his store to take a break from the sun. He has been offering his customers water to stay hydrated.

“Every time you go out there, it’s the worst,’’ Neally said, showing signs of perspiration as he sat inside. “The more water I drink, the more I sweat.’’

Keeping cool was a big problem for 36,000 NSTAR customers in Somerville, Charlestown, and Cambridge who were left without power for two hours yesterday as crews performed maintenance on a transformer. The wider interruption was corrected after about a half-hour, but other outages persisted for up to two more hours, said Michael Durand, a utility spokesman.

The effects of summer heat were of special concern on the corner of Fleet and Hanover streets in the North End, where Dee Prior, 55, packaged bouquets in the 1-800-FLOWERS store.

In this weather, air-conditioning in the shop and the car is an absolute necessity to keep flowers alive. Delivery vehicles cannot park more than a few steps from the store or petals will wilt by the time they reach the car.

“The flowers, they’re like us,’’ Prior explained, wrapping yellow blossoms in tissue paper. “This time of year, they don’t do well outside.’’

Globe correspondents Matt Byrne and Jaime Lutz contributed to this report. Powers can be reached at mpowers@globe.com.